Single phosphor for creating white light with high luminosity and high CRI in a UV LED device

ABSTRACT

There is provided a white light illumination system. The illumination system includes a radiation source which emits either ultra-violet (UV) or x-ray radiation. The illumination system also includes a luminescent material which absorbs the UV or x-ray radiation and emits the white light. The luminescent material has composition A 2-2x Na 1+x E x D 2 V 3 O 12 . A may be calcium, barium, strontium, or combinations of these three elements. E may be europium, dysprosium, samarium, thulium, or erbium, or combinations thereof. D may be magnesium or zinc, or combinations thereof. The value of x ranges from 0.01 to 0.3, inclusive.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to an illumination system whichprovides white light illumination. More particularly, it relates to anillumination system which provides illumination using an ultra-violet(UV) or x-ray radiation emitting device and a luminescent material whichconverts the UV radiation or x-rays to white light.

[0002] A luminescent material absorbs energy in one region of theelectromagnetic spectrum and emits radiation energy in another region ofthe spectrum. Typically, the energy of the photons emitted is lower thanthe energy of the photons absorbed. A luminescent material in powderform is commonly called a phosphor, while a luminescent material in theform of a transparent solid body is commonly called a scintillator.

[0003] Most useful phosphors and scintillators emit radiation in thevisible portion of the spectrum in response to the absorption ofradiation which is outside the visible portion of the spectrum. Thus,the phosphor converts electromagnetic radiation to which the human eyeis not sensitive into electromagnetic radiation to which the human eyeis sensitive. Most phosphors are responsive to more energetic portionsof the electromagnetic spectrum than the visible portion of thespectrum. Thus, there are phosphors and scintillators which areresponsive to ultraviolet light (as in fluorescent lamps), electrons (asin cathode ray tubes) and x-rays (as in radiography).

[0004] Two broad classes of luminescent materials are recognized. Theseare self-activated luminescent materials and impurity-activatedluminescent materials.

[0005] A self-activated luminescent material is one in which the purecrystalline host material, upon absorption of a high energy photon,elevates electrons to an excited state from which they return to a lowerenergy state by emitting a photon. Self-activated luminescent materialsnormally have a broad spectrum emission pattern because of therelatively wide range of energies which the electron may have in eitherthe excited or the lower energy states. Thus, excited electrons emitphotons with a fairly wide range of energies during the transition fromexcited to lower energy state, the energy of the emitted photondepending on the particular energies the electron has before and afterits emissive transition.

[0006] An impurity activated luminescent material is normally one inwhich a non-luminescent host material has been modified by including anactivator species which is present in the host material in a relativelylow concentration, such as in the range from about 200 parts per millionto 1,000 parts per million. However, some materials require several moleor atomic percent of activator ions for optimized light output. With animpurity activated luminescent material, the activator ions may directlyabsorb the incident photons or the lattice may absorb the incidentphotons and transfer the absorbed photon energy to the activator ions.

[0007] Alternatively, if the photon is absorbed directly by theactivator ion, the photon raises an electron of the activator ion to anexcited state. These electrons, in returning to their lower energystate, emit a photon of luminescent light.

[0008] When a host lattice absorbs the incident photon (i.e. theexcitation energy) and transfers it to the activator ion, the hostlattice acts as a sensitizer. The host lattice may also be doped withsensitizer atoms. The sensitizer atoms absorb the incident photon eitherdirectly, or from the host lattice, and transfer it to the activatorion.

[0009] Typically, it is desirable that the light from a lamp providewhite light, so that light from the sun, a natural light source, isimitated. The sun is a black body radiator, and thus its radiationemission spectrum obeys Planck's equation:

E(λ)=Aλ ⁻⁵/(exp(B/T _(c))−1).

[0010] E(λ) is the amount of light emitted at wavelength λ, T_(c) is thecolor temperature of the black body, and A and B are constants. As thetemperature, T_(c), of the black body increases, the wavelength of thehighest intensity emission will decrease. Thus, as the temperature of ablack body with a peak intensity emission in the red is increased, thepeak intensity emission will shift toward the blue. Although the highestintensity emission shifts in wavelength (and therefore color) with achange in temperature, the color of the broad spectrum of light emittedfrom a black body is considered to be white.

[0011] In contrast to the broad spectral range of light emitted from ablack body radiator, the range of wavelengths of light emitted from awhite light luminescent lamp may consist of only a couple of narrowemission bands, each band with a narrow range of wavelengths. Thesenarrow emission bands may be seen as white light because, in general,the color of any light source may be matched by using a mixture of threeprimary colors. White light, for example, may be generated by mixingblue and orange light, or blue, green, and red light, or othercombinations.

[0012] Because any real color may be matched by a combination of othercolors, it is possible to represent any real color with color pointcoordinates x and y in a C.I.E. chromaticity diagram as shown in FIG. 1.The C.I.E. specification of colors and chromaticity diagrams arediscussed, for example, in a textbook by K. H. Butler, “Fluorescent LampPhosphors, Technology and Theory” (Penn. State U. Press 1980), pages98-107, which is incorporated by reference. The color point coordinatesof any real color are represented by a point located within the regionbounded by the curved line representing spectral colors from extreme redto extreme violet, and the line directly between extreme red and extremeviolet. In FIG. 1, the spectral curved line is marked at certain pointsby the wavelength (in nm) corresponding to that color point.

[0013] The color points corresponding to a black body at varioustemperatures are given by the black body locus (BBL). Because the coloremitted from a black body is considered to be white, and white light isgenerally desirable for a lamp, it is generally desirable that colorpoint of the light emitted from the luminescent material of aluminescent lamp fall on or near the BBL. A portion of the BBL is shownin FIG. 1 with several color temperature points highlighted on the BBL.

[0014] Another measure of the whiteness of the light emitted from alight source is given by the color rendering index (CRI) of the lightsource. A CRI of 100 is an indication that the light emitted from thelight source is similar to that from a black body source, i.e., white.

[0015] Currently, commercial systems are available which can providevisible white light illumination using a light emitting diode (LED)combined with a phosphor. For example, one commercial system includes ablue light emitting diode of InGaN semiconductor combined with aY₃Al₅O₁₂—Ce³⁺ (YAG-Ce³⁺) phosphor. The YAG-Ce³⁺ phosphor is coated onthe InGaN LED, and a portion of the blue light emitted from the LED isconverted to yellow light by the phosphor. Another portion of the bluelight from the LED is transmitted through the phosphor. Thus, thissystem emits both blue light emitted from the LED, and yellow lightemitted from the phosphor. The mixture of blue and yellow emission bandsare perceived as white light by an observer with a CRI in the middle 80sand a color temperature, T_(c), that ranges from about 6000 K to about8000 K. The preferred color temperature of the white light illuminationsystem will depend upon the particular application and preference of theuser.

[0016] However, the cerium doped YAG phosphor suffers from severaldisadvantages. First, the cerium doped YAG phosphor system excited witha blue LED requires precise control of the cerium concentration in orderto emit white light, i.e., light with a color point on or near the BBL.Second the color of the light output of the blue LED/YAG phosphor systemis sensitive to the phosphor thickness. Third, the cerium doped YAG hasa low efficiency and a yellow color output with excitation from aradiation source with wavelengths in the UV.

[0017] To maintain white light emitted from the blue LED and ceriumdoped YAG phosphor system, the cerium concentration must be controlled.A deviation in cerium concentration from the desired concentration mayresult in an undesired deviation from white in the color of the lightgenerated by the system. While the cerium concentration may affect thecolor of the light generated by the system, the thickness of thephosphor material, as discussed below, is a more important considerationand requires careful control for generating good quality white light.

[0018] The white light is generated by a mixture of the yellow lightemitted by the phosphor and the blue light emitted by the LED andtransmitted through the phosphor. Thus, the color output of the systemwill be very sensitive to the thickness of the phosphor material. As thethickness increases, more blue light is absorbed by the phosphor insteadof being transmitted through. The light emitted by the system will thenhave a stronger yellow component emitted from the phosphor as comparedto the blue component transmitted through. The resulting light will thusappear yellowish as the thickness of the phosphor material deviatesabove the desired thickness. Likewise, the light emitted by the systemwill appear bluish if the phosphor thickness deviates below the desiredthickness.

[0019] Furthermore the cerium doped YAG phosphor does not work well withUV excitation. Specifically, the cerium doped YAG phosphor system has apoor UV efficiency. Furthermore, since blue radiation transmitted fromthe LED is required to produce white light, such white light outputcannot be achieved using YAG-Ce³⁺ and a UV emitting LED.

[0020] Another known white light illumination system which employsluminescent materials uses a blend of phosphors, each phosphor havingdifferent emission bands. The different emission bands together generatewhite light illumination. This system requires more than a singlephosphor to generate the white light illumination, and is complicated tomanufacture.

BRIEF SUMMARY OF THE INVENTION

[0021] In view of the foregoing, it would be desirable to provide anillumination system that avoids or reduces the above mentioned problems.

[0022] In accordance with one aspect of the present invention, there isprovided an illumination system. The illumination system comprises aradiation source which emits ultra-violet (UV) or x-ray radiation. Theillumination system also comprises a luminescent material which absorbsthe UV or x-ray radiation from the radiation source and emits visiblelight. The luminescent material has a compositionA_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂. A comprises at least one of calcium,barium, and strontium. E comprises at least one of the rare earths ofeuropium, dysprosium, samarium, thulium, and erbium. D comprises atleast one of magnesium and zinc. The value of x is in the range from0.01 to 0.3 inclusive.

[0023] In accordance with another aspect of the present invention, thereis provided a method for converting ultra-violet (UV) or x-ray excitingradiation to visible light. This method comprises directing the excitingradiation from the radiation source to a luminescent material. Theluminescent material comprises composition A_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂.A comprises at least one of calcium, barium, and strontium. E comprisesat least one of europium, dysprosium, samarium, thulium, and erbium. Dcomprises at least one of magnesium and zinc. The value of x is in therange from 0.01 to 0.3 inclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a chromaticity diagram showing a black body locus (BBL);

[0025]FIG. 2 shows a schematic of an illumination system according to anembodiment of the present invention;

[0026]FIG. 3 shows an excitation spectra of a luminescent materialaccording to an embodiment of the present invention;

[0027]FIG. 4 is a chromaticity diagram showing the color coordinatepoints of a luminescent material according to an embodiment of thepresent invention;

[0028]FIGS. 5a-5 c are side cross sectional views of an LED illuminationsystem in accordance with exemplary embodiments of the presentinvention;

[0029]FIG. 6 is a side cross sectional views of a fluorescent lampillumination system in accordance with an exemplary embodiment of thepresent invention;

[0030]FIG. 7 is a side cross sectional view of a plasma displayillumination system in accordance with an exemplary embodiment of thepresent invention;

[0031]FIG. 8 is a side cross sectional view of an x-ray detection systemin accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] It would be desirable to provide a system that provides whitelight illumination from a single luminescent material. It would also bedesirable to provide an illumination system that provides white light,where the concentration of the activator ion and the thickness of theluminescent material need not be precisely controlled. It would also bedesirable to provide an illumination system which responds to excitationenergies in the UV and x-ray region of the electromagnetic spectrum.

[0033] The present inventors have recognized that the problems of theprior art may be reduced or overcome by utilizing a single phosphorwhich preferably emits white light in response to exciting radiationbeyond the visible. The color of the light output of such a system wouldnot be strongly sensitive to the variations in the thickness of theluminescent material.

[0034]FIG. 2 shows an illumination system according to one embodiment ofthe present invention. The illumination system includes a radiationsource 1. The source 1 may be, for example, a light emitting diode(LED), a lamp, a laser, or some other source of radiation. The radiationsource may be any source which emits radiation which excites theluminescent material 3 of the present invention, and thus causes theluminescent material 3 to emit visible light. Preferably, the radiationsource emits radiation at less than 400 nm, such as UV or x-rayradiation or combinations thereof. Most preferably, the radiation is UVradiation.

[0035] The radiation source 1 emits radiation 2 towards a luminescentmaterial 3. The luminescent material 3 absorbs the exciting UVradiation, and emits luminescent light 4 in the visible range of thespectrum. Preferably, the luminescent light 4 is white in color.

[0036] The luminescent material 3 according to the preferred embodimentof the present invention has the compositionA_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂. A may comprise calcium, barium, strontium,or combinations of these three elements. E may comprise a rare earth,such as europium, dysprosium, samarium, thulium, or erbium, orcombinations thereof. Preferably, the main rare earth activator is Eu.Of course, even if Eu is the main rare earth activator, smaller amountsof other mentioned rare earth ions may also be present. D may comprisemagnesium or zinc, or combinations thereof. The value of x preferablyranges from 0.01 to 0.03 inclusive, i.e., 0.01≦x≦0.3. The luminescentmaterial 3, as synthesized, has a cubic garnet crystal structure. Therare earth free host structure, A₂NaD₂V₃O₁₂, is a self-activatedmaterial with a characteristic broad spectrum emission pattern. The hostabsorption typically occurs at wavelengths less than about 400 nm. Whilethe present inventors do not wish to be bound by any particular theoryas to the mechanism for host absorption and emission, the presentinventors believe that the host lattice absorption is due to thevanadate group [VO₄]³⁻. In the case of a host structure of Ca₂NaMg₂V₃O₁₂, and excitation radiation of 370 nm, the host structureemission spectrum is a broad band centered at around 530 nm.

[0037] The rare earth atoms of Eu, Dy, Sm, Tm, and/or Er act as impurityactivators when introduced in the host structure. The Eu, Dy, Sm, Tm,and/or Er doped luminescent material 3 is simultaneously aself-activated material, with the host structure providing luminescence,and an impurity activated luminescent material, with luminescence fromthe impurity activators within the host structure. The emission spectrumfrom the luminescent material 3 includes both the broad emission fromthe host structure and the much narrower emission from the rare earthimpurity activators, typically centered at a longer wavelength than thebroad emission.

[0038] The present inventors believe that the host structure acts assensitizer, absorbing the excitation energy from the UV source andtransferring this energy to the rare earth ions of either Eu, Dy, Sm,Tm, or Er, which emit the energy in the form of light radiation. In thecase of Eu in a Ca₂Na Mg₂V₃O₁₂ host structure, light is emitted in abright red line emission at 610 nm.

[0039] The emission spectrum for Ca_(2-2x)Na_(1+x)Eu_(x)Mg₂V₃O₁₂ for 370nm UV excitation, with x equal to 0.03 is shown in FIG. 3. The broademission peak centered at around 530 nm is believed to result from thehost structure emission. The much narrower red emission at 610 nm andtwo smaller satellite lines are believed to be due to Eu activatoremission. The overall emission spectrum spans the visible range andproduces a white light emission. This well balanced emission spectrumproduces a white field with a color rendering index (CRI) of 87 and aluminosity of 354 lumens per optical watt. The color coordinates(x=0.36, y=0.40) on the C.I.E. chromaticity diagram of this emissionlight are near the BBL correspond to a color temperature of 4670 K.

[0040] Dy may be substituted in whole or in part for Eu in the hoststructure. Dy emits in the yellow instead of in the red. However, likeEu, the concentration of Dy may be adjusted so that the luminescentmaterial 3 produces a white color when excited with the UV source 1. Sm,Tm, or Er may also be included as rare earth activators.

[0041] When the concentration of Eu in the luminescent material 3 isvaried from x=0.03, the color coordinates of the light emitted from theluminescent material change. However, advantageously, the colorcoordinates can be made to roughly follow the BBL. In other words,although the color coordinates change when the concentration of Eu ischanged, the light emitted remains generally white, albeit at adifferent color temperature, Tc. For example, as the concentration of Euis increased from 0.03, the color coordinate, x, increases, thecoordinate y decreases slightly, and the emitted light remains generallywhite, although the color temperature, Tc, decreases. Thus, a range ofwhitish colors can be attained from a single material.

[0042] Furthermore, because the luminescent material of the presentinvention contains more than one component which affects the emissionspectra of the material, the composition of the luminescent material maybe readily adjusted so that when the concentration of Eu is changed, thecolor of the light emitted from luminescent material will closely followthe BBL, i.e., remain white. This effect is illustrated in FIG. 4. Theopen circle at the color coordinate (x=0.36, y=0.40) corresponds to aeuropium concentration of 0.03. The arrow from that open circleindicates the direction of the change of the color coordinates forincreasing Eu concentration. As can be seen, the arrow point generallyin the direction of increasing x. In this case, Dy may be added to causethe color of the emitted light to move closer to the BBL, and thus to bewhiter.

[0043] The emission peak due to the host structure may also be changedto compensate for a change in Eu concentration so that the color of theoverall emission remains white. The emission peak due to the hoststructure can be changed by the changing the composition of the hoststructure, i.e., by replacing all or part of the Ca with Ba or Sr, or byreplacing all or part of the Mg with Zn.

[0044] Because the luminescent material of the present invention mayproduce white light simply by emitting luminescent radiation, the lightemitted by the present invention is not strongly dependent upon thethickness of the luminescent material. Thus, the present invention hasadvantages over prior art systems which required light from the excitingradiation source in addition to the luminescent light to produce whitelight.

[0045] The luminescent material may comprise a phosphor or ascintillator. The phosphor form of the luminescent material may be made,for example, by any ceramic powder method, such as a liquid phase (flux)method or a solid state method. Preferably, the method of making thephosphor comprises the following steps. First, the starting compounds ofthe phosphor material are mixed in a crucible or another suitablecontainer, such as a ball mill to form a first powder. For example, thestarting materials may be blended using a ball mill with ZrO₂ or yttriumtoughened zirconia milling media. The preferred starting phosphorcompounds comprise oxides, carbonates, hydroxides, nitrates or oxalatesof the metal constituents. For example, to formCa_(2-2x)Na_(1+x)Eu_(x)Mg₂V₃O₁₂, stoichiometric amounts of calciumcarbonate (CaCO₃), sodium carbonate (NaHCO₃), ammonium vanadate (NH₄VO₃or V₂O₅) europium oxide Eu₂O₃, and magnesium carbonate (MgCO₃,4MgCO₃.Mg(OH)₂.4H₂O) or magnesium oxide (MgO) may be mixed in thecrucible or ball mill.

[0046] The blended first powder is then fired in a furnace for betweenabout 5 and 25 hours at 600 to 1000° C. A flux may be added to the firstpowder to promote the sintering process. The sintered body is thenmilled to form a second powder. Preferably, the second powder is milleduntil it has a desired median particle size with a narrow particledistribution. The second powder is preferably milled in propanol orwater as the milling media or liquid and subsequently dried. However,other milling media, such as methanol, for example, may be used instead.

[0047] The second powder is then coated onto the desired substrate, suchas an LED, display screen or a lamp bulb. Preferably, a suspension ofthe second powder and a liquid is used to coat the substrate. Thesuspension may contain a binder in a solvent. Preferably, the bindercomprises an organic material, such as nitrocellulose, in a solvent suchas butyl acetate, amyl acetate, methyl propanol or propylene glycolmono-methyl ether acetate at a 90-95% level with 1-2% denatured ethanol.The binder enhances the adhesion of the powder particles to each otherand to the some substrates. However, the binder may be omitted tosimplify processing, if desired. After coating, the suspension is driedand may be heated to evaporate the binder further.

[0048] The scintillator form of the luminescent material may be made byany scintillator fabrication method. For example, the scintillator maybe formed by Czochralski, float zone, and crystal growing methods may beused.

[0049]FIGS. 5a-5 c, 6, and 7 show specific embodiments of theillumination system according to the present invention. FIG. 5a is anillumination system according to one aspect of the invention using alight emitting diode (LED) as a UV source. The illumination systemincludes an LED chip 10, and leads 12 electrically attached to the LEDchip. The leads 12 provide current to the LED chip 10 and thus allow theLED chip 10 to emit radiation. The radiation emitted by the LED chip 10is in the UV region of the electromagnetic spectrum, and the wavelengthof this radiation is preferably less than about 400 nm. For example, theLED chip 10 may emit at about 370 nm.

[0050] The LED chip 10 is encapsulated within a shell 14 which enclosesthe LED chip and encapsulant material 16. The encapsulant material maybe, for example, an epoxy or a polymer material such as silicone. Theillumination system of this embodiment of the present invention alsoincludes a luminescent material 3, adjacent the LED chip 10. If theluminescent material 3 is a phosphor, the luminescent material 3 may beformed over the LED chip 10, for example, by coating the light emittingsurface of the chip with the phosphor. If the luminescent material 3 isa solid scintillator, the luminescent material may be affixed to or overthe light emitting surface of the LED chip 10. Both the shell 14 and theencapsulant 16 should be transparent to allow visible light to betransmitted through those elements. The shell 14 may be, for example,glass or plastic.

[0051] The luminescent material 3 covers the LED chip 10, and thus UVlight emitted by the LED chip is incident upon the luminescent material3. The luminescent material 3 preferably has the compositionA_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂. A may comprise calcium, barium, strontium,or combinations of these three elements. E may comprise one of the rareearths europium, dysprosium, samarium, thulium, and erbium, andcombinations thereof. D may comprise magnesium or zinc. The value of xmay be in the range from 0.01 to 0.3 inclusive, i.e., 0.01≦x≦0.3.Preferably the composition of the luminescent material 3 is set so thatthe light emitted by the luminescent material is white light, asdiscussed above.

[0052]FIG. 5b shows an alternative embodiment to that of FIG. 5a. Theelements in the embodiment of FIG. 5b are the same as that of FIG. 5a,except that in the embodiment of FIG. 5b the luminescent material 3 isinterspersed within the encapsulant material 16, instead of being formedover the LED chip 10. The luminescent material 3 in FIG. 5b, preferablyhas the same composition formula as that for the material in FIG. 5a,i.e., A_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂. In this embodiment of FIG. 5b, theluminescent material 3 may interspersed within the encapsulant material16, for example, by mixing the luminescent material 3 in powder form (aphosphor) with the encapsulant material 16. For example, the luminescentmaterial 3 may be added as a powder to a polymer precursor, and then thepolymer precursor may be cured to solidify it.

[0053]FIG. 5c shows another alternative embodiment to that of FIG. 5a.The embodiment of FIG. 5c is the same as that of FIG. 5a, except that inthe embodiment of FIG. 5c the luminescent material 3 is coated on theshell 14, instead of being formed over the LED chip 10. The luminescentmaterial 3 is preferably coated on the inside surface of the shell 14,although the luminescent material may be coated on the outside surfaceof the shell 14. As another alternative, the shell 14 may be made of theluminescent material 3 in scintillator form. The luminescent material 3in FIG. 5c preferably has the same composition formula as that for thematerial in FIG. 5a, i.e, A_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂. Of course, theembodiments of FIGS. 5a-5 c may be combined and the luminescent materialmay be located in any two or all three locations.

[0054]FIG. 6 is an illumination system according to another embodimentof the invention where the system comprises a fluorescent lamp 30. Thefluorescent lamp 30 includes a bulb or cover 32 filled with a gas. Theluminescent material 3 is formed as a phosphor on the interior surfaceof the bulb 32. The fluorescent lamp 30 also includes plural cathodes orgas discharge electrodes 36 and a lamp base 38. Alternatively, theluminescent material 3 may be coated on the outside surface of the bulb32, or on a separate envelope containing the gas. The bulb 32 ispreferably made of glass. Other appropriate transparent materials, suchas plastics, may also be used. The gas, such a mercury, emitsultraviolet radiation when a potential is applied to the cathode 36through the base 38. The luminescent material 3 absorbs the incident UVradiation from the gas and preferably emits white light. The luminescentmaterial 3 in FIG. 6 preferably has the same composition formula as thatfor the material in FIG. 5a, i.e, A_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂.

[0055]FIG. 7 is an illumination system according to another embodimentof the invention where the system is a plasma display device 40. Theplasma display device comprises a transparent display screen 42, anopaque housing 44, a gas envelope 46, an array of gas dischargeelectrodes 48 (only one electrode is shown for clarity) and a controldevice 50, such as a transistor. The luminescent material 3 may beformed on the interior or exterior surface of the gas envelope 46 or onthe interior surface of the screen 42. When the control device 50applies a potential to electrode 48, the electrode 48 creates alocalized plasma discharge in the gas contained in the envelope 46. Thelocalized plasma emits UV radiation that is absorbed by an adjacentportion of the luminescent material 3. The irradiated portion of theluminescent material then emits white light through the screen 42. Animage may be formed on the screen 42 by controlling the application ofthe potential to different electrodes 48 of the electrode array.

[0056] The luminescent material 3 of the present invention may be usedin an x-ray detection system such as a computed tomography (CT) scanningsystem, as shown for example in FIG. 8. The CT scanning system is usedto obtain cross sectional images of the human body. In a CT scanningsystem, an X-ray source, such as an X-ray tube 41 rotates in a circleabout the patient 43. An X-ray detector 42 is placed on the oppositeside of the patient 43. The detector 42 rotates synchronously with theX-ray source about the perimeter of the circle. The detector comprisesthe luminescent material 3 in scintillator form optically coupled to aphotodiode or another type of photodetector. Alternatively, the detector42 may comprise the luminescent material in phosphor form coated on atransparent substrate and optically coupled to a photodiode or anothertype of photodetector. The luminescent material 3 in FIG. 8 preferablyhas the same composition formula as that for the material in FIG. 5a,i.e, A_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂.

SPECIFIC EXAMPLE

[0057] A Ca_(1.94)Na_(1.03)Eu_(0.03)Mg₂V₃O₁₂ phosphor was made by thefollowing method. Stoichiometric amounts of oxide and carbonate startingmaterials (CaCO₃, NaHCO₃, NH₄VO₃, Eu₂O₃, and MgO) were ball milled forone hour. The resulting mixture was then fired at 800° C. in air for 10hours to form Ca_(1.94)Na_(1.03)Eu_(0.03)Mg₂V₃O₁₂. The phosphor appearedwhite. The luminescence spectrum of the resulting phosphor is shown inFIG. 3 for 370 nm excitation. The chromaticity coordinates of thephosphor were (x=0.36, y=0.40) which corresponds to white light near theBBL with a CRI of 87 and a T_(c)=4670K. The phosphor had a luminosity of354 lumens per watt.

[0058] The preferred embodiments have been set forth herein for thepurpose of illustration. However, this description should not be deemedto be a limitation on the scope of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the claimedinventive concept.

[0059] All of the texts which are mentioned above are incorporatedherein by reference.

1. An illumination system comprising: a radiation source which emitsultra-violet (UV) or x-ray radiation: and a luminescent materialabsorbing the UV or x-ray radiation from the radiation source andemitting visible light, where the luminescent material comprises acomposition of matter comprising A_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂ wherein Acomprises at least one of calcium, barium, and strontium; E comprises atleast one of europium, dysprosium, samarium, thulium, and erbium; Dcomprises at least one of magnesium and zinc; and 0.01≦x≦0.3.
 2. Theillumination system of claim 1, wherein the radiation source furthercomprises a UV emitting light emitting diode (LED) and furthercomprising: an encapsulant encapsulating the LED; and a shell enclosingthe encapsulant and the LED.
 3. The illumination system of claim 2,wherein the luminescent material is a powder interspersed within theencapsulant.
 4. The illumination system of claim 2, wherein theluminescent material is adjacent the LED.
 5. The illumination system ofclaim 2, wherein the luminescent material is coated on the shell.
 6. Theillumination system of claim 2, wherein the radiation source furthercomprises a UV emitting light emitting diode (LED) and the luminescentmaterial is adjacent the LED.
 7. The illumination system of claim 1,wherein the illumination system comprises a fluorescent lamp comprising:a transparent bulb enclosing electrodes and a gas radiation source whichemits UV radiation when a potential is applied to the electrodes.
 8. Theillumination system of claim 7, wherein the luminescent material iscoated on an inside surface of the transparent bulb.
 9. The illuminationsystem of claim 7, wherein the luminescent material is coated on theoutside surface of the transparent bulb.
 10. The illumination system ofclaim 1, wherein the illumination system comprises a plasma displaysystem comprising: a plurality of electrodes; a control device whichapplies a potential to the electrodes; and a display screen adjacent theluminescent material, wherein the luminescent material emits the visiblelight through the display screen; and wherein the radiation sourcecomprises a gas which emits UV radiation when a potential is applied tothe electrodes.
 11. The illumination system of claim 1, wherein Acomprises calcium, E comprises europium, and D comprises magnesium. 12.The illumination system of claim 2, wherein A comprises calcium, Ecomprises europium, and D comprises magnesium.
 13. The illuminationsystem of claim 7, wherein A comprises calcium, E comprises europium,and D comprises magnesium.
 14. The illumination system of claim 10,wherein A comprises calcium, E comprises europium, and D comprisesmagnesium.
 15. The illumination system of claim 1, wherein the visiblelight is white light.
 16. The illumination system of claim 1, whereinthe radiation source comprises a UV emitting light emitting diode (LED)and the visible light is white light.
 17. The illumination system ofclaim 1, wherein the radiation source comprises an x-ray source, theluminescent material is a scintillator and wherein the illuminationsystem comprises an x-ray detection system comprising: a photodetectordetecting the visible light.
 18. A method for converting ultra-violet(UV) or x-ray exciting radiation to visible light comprising: directingthe exciting radiation from a radiation source to a luminescentmaterial, wherein the luminescent material thereby emits visible light;and wherein the luminescent material comprises a composition comprisingA_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂ wherein A comprises at least one ofcalcium, barium, strontium; E comprises at least one of europium,dysprosium, samarium, thulium, and erbium; D comprises at least one ofmagnesium and zinc; and 0.01≦x≦0.3.
 19. The method of claim 18, whereinA comprises calcium, E comprises europium, and D comprises magnesium.20. The method of claim 18, wherein the visible light is white light.21. The method of claim 18, wherein the radiation source comprises anLED and the luminescent material comprises a phosphor over the LED. 22.A method for converting UV or x-ray radiation to white light comprising:directing the exciting radiation from a radiation source to a singleluminescent material; and emitting white light from the singleluminescent material.
 23. The method of claim 22 wherein the singleluminescent material comprises a phosphor composition comprisingA_(2-2x)Na_(1+x)E_(x)D₂V₃O₁₂ wherein A comprises at least one ofcalcium, barium, strontium; E comprises at least one of europium,dysprosium, samarium, thulium, and erbium; D comprises at least one ofmagnesium and zinc; and 0.01≦x≦0.3; and wherein the radiation source isan InGaN light emitting diode (LED).
 24. The method of claim 23, whereinthe color of the emitted light is independent of the thickness of theluminescent material.